The present invention relates in general to a sheet feeding apparatus, and in particular to an apparatus of the type having a displaceable sheet separator and means for positioning the sheet separator according to the format of sheets when a new stack of sheets of different format is applied to the feeding apparatus.
German published patent application No. 27 50 105 discloses a device by means of which stop gauges of sheet processing machines are adjustable according to the size of sheets under process. In this known device, among other measures, the stop gauges are fixedly arranged in the direction of advance of the sheet on the sheet separator and are displaced together with the sheet separator by means of power-driven screw spindles, for example. The desired measure of the format corresponding to the processed sheets is determined by means of a feeler head arranged on a special measuring rod.
It is true that by means of these prior-art measures a decrease in manual operation is achieved, but this known device requires complicated technology for the positioning devices, including electronic preselection devices with digital read-out. Another disadvantage of this known device is the fact that upon the entry of data pertaining to a new format of processed material, the sheet separator travels to the given position without regard to the actual position of the stack. In practice, the outer contours of the stack and thus the position of the stack deviate frequently from the format of individual sheets, and therefore a difference between a theoretical position of the stack and the actual position thereof frequently occurs. In this event, time-consuming position corrections are necessary. Moreover, during such corrections the possibility exists that the upper layers of the stack may be damaged by stop gauges, inasmuch as the stack position variations occur most frequently as an enlargement of the sheet format.
In the GDR Pat. No. 107,638 a method of adjusting elements of a printing machine according to the format of the processed sheets is disclosed. In this known method, a sensing device is also moved synchronously with the movement of elements being adjusted relative to an edge of the sheet, whereby the drives for the elements and for the sensing device are disconnected when the latter abuts against the edge of the sheet stack. This method solves the problem of adjusting the position of a sheet separator according to a new format of the sheets only partially; it presumes again that in a format exchange the sheet separator be brought into a zero or starting position from which it can be displaced always in one positioning direction only. If the adjustment into the starting position after the feeding of a new sheet stack be incidentally omitted, breakage of machine elements and damage of the sheet stack might result.